Human telomere quadruplex: refolding and selection of individual conformers via RNA/DNA chimeric editing

The conformation of the guanine quadruplex formed by the human telomere (HT) repeat in solutions containing physiological concentrations of K+ ions has been a topic of intensive investigation during the past several years. Of particular interest are the directionality of the overall folding pattern, i.e., parallel, antiparallel, or a combination of these two modes, and the alternation, if any, of the glycosidic bond conformation between syn and anti. An additional issue involves resolving mixtures of conformations when more than one species is present. We approach these questions using selective substitution of riboguanosine, rG, for deoxyriboguanosine, dG. Using a combination of circular dichroism, gel electrophoresis, equilibrium ultracentrifugation, and imino proton NMR, we are able to show that these modifications can yield sequences which fold into parallel or antiparallel conformations consisting of one or two strands. We also demonstrate that chimeric editing of the HT sequence permits isolating one of two conformational isomers existing in solution in the presence of KCl. The ability to engineer and control quadruplex folding motifs illustrated here with HT may prove useful more generally for a variety of quadruplex-forming sequences.